Notes - L - 1, Chemical Reactions and Equations

Chapter – 1 (NOTES)

Chemical Reactions and Equations

Chemical Reactions : -

• Process in which new substances with new properties are formed from one or more substances is called Chemical Reactions.
• The substance which take part in chemical reaction are called Reactants
• The substance which are formed in a chemical reaction are called Products.

Examples of Chemical Reaction : -

• Digestion of food
• Respiration
• Rusting of Iron
• Burning of Magnesium ribbon
• Formation of curd

Chemical Reaction involves : -

• Change in state
• Change in colour
• Change in temperature
• Evolution of gas

Chemical Equations : -

• A chemical reaction can be represented by chemical equation. It involves uses of symbol of elements or chemical formula of reactant and product with mention of physical state.
• The necessary condition such as temperature, pressure or any catalyst should be written on arrow between reactant and products.

Example of Chemical Equation :-

Reaction - Magnesium is burnt in air to form magnesium oxide.

The word equation of this reaction

Magnesium  +  Oxygen  →  Magnesium Oxide

The chemical equation of this reaction

Mg  +  O₂  →  MgO

Balancing the Chemical Equation

Balancing the chemical equation is necessary because : -

Law of conservation of Mass. Mass can neither be created nor be destroyed in any chemical reaction.

So number of elements involved in chemical reaction should remain same at reactant and product side.

How to Balance the Chemical Equation ?

Please watch this video for easy way to balance the chemical equation

Another way of Balancing Chemical Equation is

Stepwise Balancing (Hit and Trial) : -

Step 1 – Write a chemical equation and draw boxes around each formula

Fe  +  H₂O  → Fe₃O₄  + H₂

Step 2 – Count the number of atoms of each element on both sides of chemical equation.

Sno.	Element	No of atoms at reactant side	No. of atoms at product side
1	Fe	1	3
2	H	2	2
3	O	1	4

Step 3 – Equalise the number of atoms of element which has maximum number by putting in front of it.

3Fe  +  4H₂O  → Fe₃O₄  + 4H₂

☆ Now all the atoms of elements are equal on both sides.

Step 5 – Write the physical states of reactants and products. Also write the necessary condition like – temperature, pressure or catalyst on arrow above or below.

Solid state – (s)

Liquid state – (l)

Gaseous state – (g)

Aqueous state – (aq)  It is a solution in which the solvent is water.

3Fe (s)  +  4H₂O (l)  → Fe₃O₄ (s) + 4H₂ (g)

Types of Chemical Reactions

1 – COMBINATION REACTION – The reaction in which two or more reactant combine to form a single product.

Examples of Combination Reactions

(i) Burning of coal 

C (s)  +  O₂ (g) → CO₂ (g)

(ii) Formation of water

2H₂ (g)  +  O₂ (g) →  2H₂O

(iii) CaO (s)  + H₂O → Ca(OH)₂ (aq)

       Quick lime               Slaked lime

Exothermic Reaction – Reaction in which heat is released along with formation of products.

Example of Exothermic Reactions

 Burning of natural gas

CH₄ (g)  +  O₂ (g)  → CO₂ (g)  +  2H₂O (g)  +  Heat

2 – DECOMPOSITION REACTION – The reaction in which a compound splits into two or more simple substances is called decomposition reaction.

There are three types of Decomposition Reaction

• Thermal Decomposition Reaction – The decomposition is carried out by heating is called Thermal Decomposition Reaction

Ex – CaCO₃ (s) → CaO (s)  + CO₂ (g)

• Electrolytic Decomposition Reaction – The decomposition is carried out by passing electricity called Electrolytic Decomposition Reaction

Ex – 2H₂O → 2H₂  +  O₂

• Photolytic Decomposition Reaction – The decomposition is carried out in presence is called Photolytic Decomposition Reaction

Ex – 2AgCl (s) → 2Ag (s)  +  Cl₂ (s)

        2AgBr (s)  →   2Ag (s)  +  Br₂ (s)

Endothermic Reaction – The reactions which require energy in the form of heat, light or electricity to break reactants are called Endothermic Reactions.

3 – DISPLACEMENT REACTION – The chemical reaction in which more reactive element displaces less reactive element from its salt solution.

Example of Displacement Reaction –

Fe (s)  +  CuSO₄ (aq) →  FeSO₄ (aq)  +  Cu (s)

When Iron nail dipped in copper sulphate solution, then the iron nail becomes brownish in colour by deposition of Cu and blue colour of CuSO₄ changes dirty green colour due to formation of FeSO₄

4 – DOUBLE DISPLACEMENT REACTION – A reaction in which new compounds are formed by mutual exchange of ions between two compounds.

Example of Double Displacement Reaction –

Na₂SO₄ (aq)  +  BaCl₂ (aq)  →  BaSO₄ (s)  +  2NaCl (aq)

In the above reaction white precipitate BaSO₄ is formed by the reaction of the ions of SO₄^2- and Ba²⁺.

5 – OXIDATION AND REDUCTION :-

Oxidation  - The addition of oxygen to substance and the removal of hydrogen form a substance is called Oxidation.

Ex –     2Cu  +  O₂   →  2CuO

Reduction – The addition of hydrogen to substance and the removal of oxygen from a substance is called Reduction.

 Ex -

The above Reaction is also called Redox Reaction

Redox Reaction –  In a reaction oxidation and reduction occur simultaneously that is called Redox Reaction  

Effects of Oxidation in Daily life : -

1 – Corrosion – When a metal is exposed to substance such as moisture, air, water etc. for some time, a layer of oxide is formed which weakens the metal and hence metal is said to be corrode

Examples of corrosion –

• Rusting of Iron
• Black coating on Silver
• Green coating on copper

Corrosion can be prevented by galvanization, electroplating or painting.

2 - Rancidity – The oxidation of fats and oils when exposed to air is known as rancidity. It leads to bad smell and bad taste of food.

Methods to prevent Rancidity –

• By adding antioxidants (nitrogen)
• Keeping food in air tight containers.